1. Field of the Invention
This invention relates to the preparation of organohalosilanes. More particularly, this invention relates to a method for increasing the yield of monoorganodichlorosilanes and/or reducing the yield of undesired monoorganotrichlorosilanes using a method referred to in the art as the "direct process" without either adversely affecting the yield of other desirable organochlorosilanes, particularly the combined yield of monoorganodichlorosilane and the corresponding diorganodichlorosilane, or generating substantial quantities of undesirable inorganic halosilanes and organotrihalosilanes.
2. Description of Relevant Art
The preparation of organohalosilanes by the reaction of an alkyl or aryl halide with silicon metal in the presence of various catalysts is known as the "direct process". The halide portion is typically chlorine, but can also be bromine or iodine.
Preparing halosilanes by reacting silicon with hydrogen chloride has been known since the work of Buff and Wohler in 1857 and Combes in 1896. Application of the direct process to the preparation of organohalosilanes was first disclosed by Rochow and his co-workers, beginning in the mid-1940's. The art describes numerous improvements to this direct process.
Rochow and Patnode, U.S. Pat. No. 2,380,996, issued Aug. 7, 1945, and Patnode, U.S. Pat. No. 2,380,997, issued Aug. 7, 1945, disclose the preparation of a contact mass for the direct process. The mass is prepared by firing a mixture of silicon, copper, or other metallic catalysts in a reducing atmosphere. Rochow and Patnode and Patnode also disclose the use of nickel, tin, antimony, manganese, silver, and titanium.
Rochow and Gilliam, U.S. Pat. No. 2,383,818, issued Aug. 28, 1945, discloses the use of contact masses comprising silicon and an oxide of copper. Also, included are copper compounds which are readily converted to the oxides, such as copper nitrate. An example of more recent art is Chapters 4 and 5 of a text entitled Organohalosilanes by R. J. H. Voorhoeve, published in 1967 by Elsevler.
For various reasons, including cost and availability of starting materials, alkylchlorosilanes, particularly methyl- and ethylchlorosilanes, have become the organohalosilanes most frequently prepared by the direct process. The present invention has therefore been limited to this class of alkylchlorosilanes using the corresponding alkyl chlorides. It should be understood that while preferred embodiments of the present invention are directed Primarily to the preparation of certain methylchlorosilanes by reacting methyl chloride and silicon, the invention is not to be so limited.
When methyl chloride, represented by the formula MeCl, and silicon metal are reacted using the catalysts and reaction conditions described in the prior art, the resultant products include but are not limited to MeHSiCl.sub.2, Me.sub.2 SiCl.sub.2, Me.sub.3 SiCl, MeSiCl.sub.3, Me.sub.2 HSiCl, HSiCl.sub.3 and SiCl.sub.4, where Me represents the methyl radical. By an appropriate selection of catalyst and reaction conditions it is possible to obtain dimethyldichlorosilane, Me.sub.2 SiCl.sub.2, as the major component, often 90 weight % or more, in the final product mixture. Methyldichlorosilane, MeHSiCl.sub.2 typically constitutes about 1 weight percent of the product under these conditions, which are designed to optimize the yield of Me.sub.2 SiCl.sub.2. The reaction product also typically contains a significant concentration of methyltrichlorosilane, MeSiCl.sub.3, which in many instances is not a desired product.
The prior art provides no teaching to enable one to increase the relative concentration of methyldichlorosilane in a direct process product mixture without either a substantial decrease in the combined yield of this product and dimethyldichlorosilane or producing substantial amounts of undesirable products, particularly methyltrichlorosilane and carbon. Carbon is especially undesirable because it collects on the silicon reaction mass and may decrease the rate of the methyl chloride/silicon reaction or cause it to stop all together.
The art teaches combining the methyl chloride with 5 weight percent or more, based on methyl chloride, of hydrogen as a means for increasing the relative yield of methyldichlorosilane in the final product, however the yield of dimethyldichlorosilane is more than correspondingly reduced and substantial quantities of undesirable organic halosilanes are produced.
For example, U.S. Pat. No. 2,380,999 which issued to Sprung et al. on Aug. 7, 1945 teaches sintering a mixture containing 90 weight percent silicon and 10 weight percent copper for one hour at 1050.degree. C. under a hydrogen atmosphere. The resultant reaction mass is then placed in a stream of ethyl chloride flowing at a rate of 80 cc. per minute. Following separation of volatile materials the liquid reaction product was found to contain 73.5 weight percent dimethyldichlorosilane, 9 percent methyltrichlorosilane, 6 weight percent trimethylchlorosilane, and "small amounts of other methylchlorosilanes". Combining the methyl chloride with nitrogen increased the yield of dimethyldichlorosilane to 86.5 percent, the only other specifically reported product being 4.0 weight percent of methyltrichlorosilane.
An article by Csakvari et al. that appeared in Acta Chim. Acad. Sci. Hung. 39 (1), 33-7 (1963) mentions prior art disclosing the use of methyl chloride/hydrogen mixtures to achieve a 20 weight percent yield of methyldichlorosilane in the final product. The types and amounts of other methylchlorosilanes produced are not disclosed.
British patent no. 1,089,726, which issued to Morozov et al. on Nov. 8, 1967 teaches obtaining up to a 65 weight percent yield of methyldichlorosilane, based on total reaction product, or up to a 16 weight percent yield of dimethylchlorosilane by adding iron, cobalt, nickel, or their salts to the silicon/copper reaction mass in a direct process and blending the methyl chloride with hydrogen. The rates of addition of hydrogen and methyl chloride are each from 0.1 to 0.5 liters per minute. The products described in the examples of this patent contain from 13 to 33 weight percent of methyltrichlorosilane.
An article by J. Rathousky et al. [Chem. Prum., 22(10), 485-8 (1972) discusses the effect of hydrogen addition on a direct process reaction of methyl chloride with a 9:1 weight ratio mixture of silicon and copper under superatmospheric pressure at a temperature of 320.degree. C. The yield of methyldichlorosilane increased by a factor of 5 and the yield of dimethyldichlorosilane decreased proportionately as the hydrogen concentration was increased from 0 to 15 mole percent of a methyl chloride/hydrogen mixture. The yield of methyltrichlorosilane remained substantially constant over the range of hydrogen concentrations investigated. The combination of methyldichlorosilane and dimethyldichlorosilane amounted to about 60 weight percent of the reaction product at all hydrogen concentrations, and the reaction product contained small amounts of dichlorosilane, trichlorosilane, silicon tetrachloride and trimethylchlorosilane.
The effect of varying amounts of hydrogen on the direct process reaction using a copper catalyst is reported by M. De Cooker et al. in the Journal of Organometallic Chemistry, 99(1975) 371-377. During the 5 experimental runs the partial pressure of hydrogen was varied from 0.55 to 0.75 atmosphere, and the combined pressure of hydrogen and the methyl chloride reactant totaled one atmosphere. The highest yield of dimethyldichlorosilane reported is 88 mole percent, and the corresponding yields of methyldichlorosilane and trimethylchlorosilane were 4 mole percent each.
The prior art also teaches combining the methyl chloride with 5 weight percent or more, based on methyl chloride, of hydrogen chloride as a means for increasing the relative yield of methyldichlorosilane in the final product, however the yield of dimethyldichlorosilane is more than correspondingly reduced and substantial quantities of undesirable inorganic halosilanes are produced. For example, U.S. Pat. No. 3,454,616, which issued to Ariga et al. on July 8, 1969 teaches reacting silicon metal with mixtures of methyl chloride and from 20 to 83 percent, based on the weight of the mixture, of hydrogen chloride. In accordance with the examples of this patent, when these gaseous mixtures are reacted with metallic silicon containing catalytic amounts of copper and nickel the reaction product contains up to 36 weight percent of methyldichlorosilane.
Depending upon reaction conditions and the molar ratio of hydrogen chloride to methyl chloride, the reaction product also contained from 2.4 to 32.5 weight percent of dimethyldichlorosilane. The product also contained traditionally undesirable products, including from 5.6 to 33 weight percent of trichlorosilane, from 14.6 to 25.2 weight percent of methyltrichlorosilane and from 0.6 to 2.3 weight percent of silicon tetrachloride.
The addition of aluminum chloride or boron trichloride in catalytic amounts to increase the amount of methyl-dichlorosilane formed by the reaction of methyl chloride and hydrogen chloride with silicon metal is taught in U.S. Pat. No. 3,109,014, which issued to Tamura et al. on Oct. 29, 1963. As in the Ariga et al. patent, excessive amounts of the undesirable inorganic halosilanes are produced.
Golubsov et al. report in the Journal of Applied Chemistry, Russian edition, 37 (7), p. 1634 (1964) that the presence of hydrogen chloride increases the yield of phenyltrichlorosilane from the reaction of chlorobenzene and silicon metal. A product containing 55 mole percent of phenyltrichlorosilane and only 0.7 mole percent of phenyldichlorosilane is reportedly obtained from the reaction of a silicon alloy with a mixture of chlorobenzene and hydrogen chloride containing 62 weight percent of hydrogen chloride. The yield of diphenyldichlorosilane is not disclosed.
L. Morozov et al. [Izvestia Akaademii Nauk SSSR Ser. Kim. (1962) (6) 941] reacted a mixture of silicon and catalytic amounts of copper oxide, zinc oxide and sodium silicate with a mixture of methyl chloride and 5 or 10 percent by volume of hydrogen chloride at a temperature of 350.degree. C. Five volume percent of hydrogen chloride produced 43 weight percent of methyldichlorosilane, 2 percent of dimethyldichlorosilane, 31 percent of methyltrichlorosilane, and 6 weight percent of silicon tetrachloride. Ten volume percent of hydrogen chloride yielded 42 weight percent of methyldichlorosilane, 8 percent of methyldichlorosilane and 9 percent of silicon tetrachloride.
The data in an article by Gorbunov et al. in the September 1970 issue of Doklady Akademi Nauk. SSR. 194, 1 (92-94) demonstrate that 35 weight percent of methyldichlorosilane, only about 10 weight percent of dimethyldichlorosilane with about the same amount of silicon tetrachloride is obtained by reacting silicon with a 3:1 weight ratio mixture of methyl chloride and hydrogen chloride at 300 degrees C.
J. Joklik et al. [Collect. Czech. Chem. Commun. (1964) 29(3) 834] disclose reacting silicon with various ratios of methyl chloride to hydrogen chloride in the presence of a copper catalyst at temperatures of 260.degree., 300.degree., and 350.degree. C. At 260.degree. C. as the mole ratio of hydrogen chloride to methyl chloride in the reactor was stream was increased from 1:6 to 1:1 the weight percent of methyldichlorosilane in the reaction product increased from 8.9 to 14 percent while the yield of trichlorosilane increased from 4.3 to 15 percent and the yield of methyltrichlorosilane increased from 15.5 to 32.9 weight percent. At 350.degree. C. using a 1:6 mole ratio of hydrogen chloride to methyl chloride in the reactor gas stream the yield of methyldichlorosilane was 6.5 weight percent, while the yields of undesirable methyltrichlorosilane, trichlorosilane and silicon tetrachloride were 35.3, 0.4 and 0.3 percent by weight, respectively. When the mole ratio of hydrogen chloride to methyl chloride was 2:3 the reaction product contained 14.1 weight percent of methyldichlorosilane, 52.4 weight percent of methyltrichlorosilane 2.9 percent of trichlorosilane and 1.2 percent of silicon tetrachloride.
Finally, the effect of diluting ethyl chloride with various levels of hydrogen chloride was reported on by Andrianov et al. in Izvestiya Akademii Nauk SSSR (Chemical Section) 10 , 1788-1794 (1962). Reacting a mixture of 92 weight percent ethyl chloride and 8 percent hydrogen chloride yielded a mixture of chlorosilanes containing 30 percent by weight of trichlorosilane, 12 percent of SiCl.sub.4, 8.8 percent of ethyldichlorosilane and 50 percent of diethyldichlorosilane.
Methods for increasing the yields of dimethyldichlorosilane while reducing the yield of methyltrichlorosilane obtained using the direct process are described in the prior art. For example, U.S. Pat. No. 4,500,724, which issued to Ward et al. on Feb. 19, 1985 teaches using specified mixtures of copper, zinc and tin to increase the yield of dimethyldichlorosilane and reduce the yield of methyltrichlorosilane in a direct process reaction. The weight ratio of methyltrichlorosilane to dimethyldichlorosilane reported in the examples of this patent is from 0.041 to 0.099.
An objective of this invention is to provide a method for increasing the yield of methyldichlorosilane in the direct process without substantially affecting the combined yield of this silane, dimethylchlorosilane and dimethyldichlorosilane or producing excessive amounts of methyltrichlorosilane or inorganic chlorosilanes such as SiCl.sub.4 and SiHCl.sub.3.